The present invention relates particularly to a fin assembly designed to increase fin surface area for projectiles.
Firing range tests of 60 mm (millimeter) solid fuel ramjet projectiles from a 120 mm gun revealed certain significant deviations from expected trajectory. Among the explanations for such behavior are: damaged fins (e.g., attributable to in-flight metal burning, or to in-bore contact with the propellant granules during the projectile release from its primer case), and insufficient fin normal force required for static stability. In an attempt to solve the problem, the area of each fin 10 (FIG. 1) ahead of the leading edge 12 of the fin was increased, as illustrated by the dashed area 15, to provide an additional normal force, viz. .DELTA..sub.1 F.sub.f, but the resulting increase in stabilization was found to be insufficient. This undesirable result is attributable to the smaller fin moment arm x.sub.1cp (of that additional force for the increased area) relative to the center of gravity (CG) of the finned projectile 17.
It was recognized that the addition of surface area 20 at the trailing edge 22 of fin 10-1 (and of the other fins 10-2, 10-3 and 10-4, the latter being hidden by the body of projectile 17 in FIG. 1), rather than at the leading edge, would be more effective because that solution would serve to produce a larger fin moment arm x.sub.2cp, and additional normal force .DELTA..sub.2 F.sub.1. However, such a solution could not be implemented because of the limitation on the distance .DELTA.x.sub.f, owing to projectile packaging constraints.
In part, the packaging constraints are better understood by reference to FIG. 2 as well as to FIG. 1. The projectile 17 is packaged for use in a conventional propellant case assembly. The case assembly includes a pusher disk 30 having a rotating band/obturator 32, and is partly inserted into the propellant charge casing 35. The set of geometrical constraints on the fin design includes the following. The fin height h (FIG. 1) is fixed because the maximum diameter d.sub.f is limited by the bore size of the gun. The fin root chord length cannot be extended beyond the base of the projectile in excess of the length .DELTA.x.sub.f (FIG. 1), because of projectile packaging considerations for the attachment to the primer case containing the propellant charge. Also, the number of fins of the projectile cannot be increased to exceed four, such as to six as a means to increase the fin surface area, because of a limitation on the number of sabot pieces permitted for positioning the projectile in the gun barrel.
It is a principal object of the present invention to increase the stabilization of finned projectiles during flight, to be able to maintain the trajectory of the projectile.
Another object of the invention is to provide a finned projectile with improved stability, and thereby with the capacity to more accurately follow a desired trajectory, without violating the many practical constraints on size, shape and number of the fins imposed on the projectile.